Symmetry-Breaking of Turbulence Structure and Position Identification in Toroidal Plasmas

Abstract

As a result of the progresses in the study of plasma turbulence, the picture of 'multiple-scale turbulence' is widely accepted, and the 'nonlinear excitation, nonlocal interaction, probabilistic transition' views have been developed. Along the paths of these progresses, importance of new problem, i.e., the symmetry-breaking of turbulence structure (such as up-down asymmetry, excitation of streamer, etc.) is gradually recognized. In this topical review article, we revisit these issues, and illuminate the possibilities that the new progresses (which will be brought about by studying the symmetry-breaking of turbulence) are expected. Examining theoretical predictions and preceding experimental achievements, new advancements, which will be realized, are explained. The experimental study of symmetry-breaking of turbulence structure ultimately requires to measure fluctuations (at all scales) over the whole plasma cross-section simultaneously. In addition, new type of demands can be imposed in measuring of fluctuations over the whole plasma cross-section simultaneously. One of such demands is the accuracy of the measurement position. Problems in this aspect are also discussed. This concise review is used to identify what will be discovered and how it will be reached in future experiments, in the subject of the symmetry-breaking of turbulence structure.

Geodesic acoustic modes in magnetic confinement devices

“…Surface location: the GAM m = n = 0 mode structure and narrow radial extent can be exploited as a means of identifying the radial location (to an accuracy to a few gyroradii) of a specific iso-magnetic surface through long-range-correlation measurements of the flow perturbation [636]. Further, on the assumption that the GAM exists only on closed flux surfaces this may also be used as a rough locator for the tokamak last closed flux-surface or separatrix position (noting that the GAM may be suppressed close to the separatrix due to increasing collisionality) [322].…”

Review of plasma turbulence experiments

“…To realize such advanced observations needs a new device and project where new advanced diagnostics, for example, tomography for plasma turbulence, can be realized, 103 – 105 ) although a physics-oriented study might be possible concerning the present-day device relevant to ITER. Such a device, named PLAsma Turbulence Observatory (PLATO), is being constructed in a project entitled ‘Plasma Turbulence Observation System for puzzling out the principles of structural formation and functional expression in turbulent plasmas’ under the support of Specially Promoted Research, 104 , 105 ) with an aim to clarify any unsolved problems concerning the concept of symmetry-breaking and global linkage.…”

The first observation of 4D tomography measurement of plasma structures and fluctuations

“…Furthermore, it has been pointed out that a turbulence asymmetry or a localization is on the magnetic field flux surface, which is reported in Refs. 6 – 8 , should considerably affect plasma transport. However, a direct and simultaneous observation of the entire structure with the disparate scale fluctuations is difficult by the conventional diagnostics, which only provide a measurement of local turbulence in a limited region of plasma cross-section 9 – 11 .…”